This invention relates to a filter, particularly for beverages, comprising a pressure vessel containing vertically spaced apart partitions for supporting respective layers of a substantially particulate filtering material.
In known filters of that kind, the filtering powder material, which usually consists of diatomaceous earth, is permitted to settle out of a suspension on the several partitions so as to form a filter bed. A pure liquid, such as water, is used as a fluid in which the filtering material is suspended. At the beginning of the filtration, that liquid must be displaced by the slurry which is to be filtered. As the pressure vessel is filled with a liquid in which the filtering material is suspended, the pressure vessel is perfectly degassed by the rising liquid. But as said liquid is displaced by the slurry to be filtered, the latter is mixed with the suspending liquid and this mixing may result in a loss of filtrate. Similar disadvantages are encoutered when the filtration has been terminated because the liquid which is required to displace the slurry from the filter bed is also mixed with the slurry or the filtrate. Besides, solids which have been retained by the filter bed may be detached therefrom.
As the filtration depends on the condition of the filter bed, fresh filtering material must be continually supplied during the filtration so that the surface of the filter bed is continually re-formed and the filter bed is maintained in a permeable condition. As the condition of such a filter bed may also be influenced by fluctuations of the flow, pressure and temperature conditions of the fluid to be filtered, it is difficult to achieve constant results by the filtration.
It is an object of the invention to avoid these disadvantages and so to improve a filter of the kind described first hereinbefore that the supply of the filtering material, in the form of a suspension, and the disadvantages involved in the supply in that form, can be eliminated and that it is ensured that the result of the filtration will not be influenced by fluctuations of the flow, pressure and temperature conditions of the fluid which is to be filtered.
This object is accomplished according to the invention by enclosing each layer of filtering material by a sheath, which is permeable to the fluid to be filtered and impermeable to the filtering material whereby a filtering package is obtained, which is engaged at its top and bottom by respective partitions and is compressed by the partitions under an at least approximately constant pressure, and passages are defined between each filtering package and the two partitions engaging the package and serve to supply the fluid to be filtered to the package and to withdraw filtrate from the package.
The sheath may consist of any known filter cloths, provided that it is permeable to the fluid to be filtered and impermeable to the filtering material contained in the package.
The pressure under which each filtering package is compressed should be sufficient to prevent a channeling in the filtering material package in response to changes of the flow rate and pressure of the fluid which is to be filtered.
Because the layer of filtering material which is enclosed by a sheath that is impermeable to the filtering material constitutes a filtering package which is compressed between the partitions engaging the filtering package at its top and bottom, the particulate filtering material will be held together under all operating conditions so that the effect of a sintered body of filtering material is obtained in spite of the fact that the filtering material is particulate. On the other hand, the disadvantages involved in the use of a sintered body of filtering material are avoided, such body being able to be regenerated only to a restricted degree because the solids of the slurry which is to be filtered wil enter the pores of the sintered body and will be so firmly retained in the pores that they cannot be removed even by very intense backwashing. Because in accordance with the invention the particulate filtering material is enclosed by a sheath, relatively large solids cannot pass through the sheath and enter the filtering material. The filtering material has such a surface texture that any solids which have reached the filtering material cannot be anchored in the latter so that all solids will be removed from the filtering material when the latter is backwashed and such backwashing will result in a perfect purification and regeneration of the filtering package. Where the solids consist of organic substances, such purification and regeneration can be ensured by a backwashing with an alkaline solution.
The filtering properties of each layer of filtering material layer will depend on the geometry of the layer and will be independent of the operating conditions because the layer will be held together under the pressure applied to the filter package. For this reason, uniform results of the filtration will be ensured. It will be understood that the use of filtering packages comprising a particulate filtering material the particles of which are immovably held under an at least approximately constant pressure which is applied will eliminate the need for supplying the filtering material in the form of a suspension in a liquid. For this reason the pressure vessel which contains the filter packages between the partitions can be degassed as a slurry to be filtered is supplied to each filtering package in the passages between the package and the partitions and flows through the filtering package and a filtrate is drained from the filtering package on the other side thereof. Because the filtering packages are vertically superimposed, the gas contained in the pressure vessel is entirely displaced by this operation. As a result, there will be no undesired draining of a mixture of filtrate and of liquid in which a filtering material was suspended and the disadvantages involved in the draining of such mixture will be avoided.
According to a further feature of the invention, a filter having a particularly simple structure will be obtained if a partition which is impermeable to filtrate is provided between, the engaged by, adjacent filter packages. A very compact filter can be obtained in this manner although it is recommended to subject the slurry to a preliminary coarse filtration in order to avoid a retention of coarse solids by the filter packages, which are particularly suitable for a fine filtration.
If the partition engaging a filtering package at its bottom consists of a sieve and is supported by pressure-resisting means on the filtrate-impermeable partition engaging the top of the next lower filtering package and a plenum chamber for solids-laden fluid is defined between the sieve and filtrate-impermeable partition, it will be possible to use the filter for a filtration in two stages, namely, a coarse filtration and a fine filtration. In such a filter the sieve will retain coarse particles, which will remain in the plenum chamber between the sieve and the next lower partition which is impermeable to filtrate, so that said coarse particles can easily be entrained out of the plenum chamber by backwashing. It is important that the sieve is supported by pressure-resistant means on the partition which engages the next lower filtering package at its top so that the required pressure can be applied to each filtering package by the associated partitions.
If the sieve consists of a perforated plate, coarse solid particles which are to be retained may be seized in the holes of the sieve and it may not be possible to remove such particles by backwashing. To ensure that the sieves will not be clogged, the undersurface of each sieve may be covered by a filter cloth, which will prevent an ingress of coarse solid particles into the holes of the sieve.
The partitions are usually mounted on a rod which is centered on the vertical central axis of the pressure vessel. When this known design is adopted, the required pressure can be applied to the filtering packages in a particularly simple manner if the partitions are axially slidably mounted on the central rod and are gripped between two pressure-applying heads, one of which is axially immovably fixed to the rod whereas the other pressure-applying head is slidably mounted on the rod and is connected to an actuator, by which an adjustable, desired pressure can be applied via the pressure heads and the partitions to the filtering packages whereas the rod acts as a tie rod.
In order to prevent a shifting of solid particles in a given filtering package and resulting changes of the geometric configuration of such package, particularly a channeling in the filtering material, the pressure applied to the packages should be at least approximately constant regardless of any changes of the temperature and pressure of the solids-laden fluid to be filtered. If the pressure applied is high in relation to the changes of the pressure of the solids-laden fluid, which changes may be due to temperature fluctuations, then it will be sufficient to apply a constant pressure to each filtering package by means of the partitions. But if the influence of the changes of the pressure of the solids-laden fluid is not negligible, then any fluctuation of the pressure of said fluid in the pressure vessel will have to be compensated by a change of the pressure applied to the filtering packages by the partitions. This can be accomplished in a very simple manner if the pressure-applying means consists of a fluid-operable cylinder, to which a predetermined fluid pressure is applied. In that case, a constant pressure will be applied to the filtering packages regardless of any fluctuations of the pressure of the solids-laden fluid and a shifting of particles in any filtering package will be prevented.
Instead of a fluid-operable cylinder, a pressure applying spring may be used as an actuator because the displacement remains negligibly small.
Particularly desirable conditions for the degassing of the pressure vessel or for the complete draining of a liquid from the pressure vessel will be obtained if, according to a further feature of the invention, the partitions and the filtering packages have the configuration of inverted hollow cones, which are open-topped. The gas contained in the pressure vessel will then be displaced upwardly by the slurry which is to be filtered owing to the buoyancy of such gas whereas no gas to be displaced can accumulate in dead spaces. On the other hand, when air is admitted to the pressure vessel, all liquid can be drained from the filter packages. If the hollow cones have included angles between 90 and 150 degrees, the distribution of the slurry over each filtering package and the flow conditions will be satisfactory.
It will be understood that the filtrate must be removed from each filtering package adjacent to the top rim of each open-topped inverted hollow cone if the degassing of the pressure vessel is to be assisted by the supply of the slurry into the vessel. A simple structure will be obtained if the passages defined between each filtering package and the partition engaging the top of the filtering package communicate with an annular manifold surrounding the outer rim of the partition. To ensure a uniform distribution of the filtrate to be drained over the surface of each filtering package, the annular manifold may be connected to a plurality of axially extending withdrawing pipes which are regularly spaced around the periphery of the vessel near the inside peripheral surface thereof.
If the passages left between each filtering package and the partition engaging the package at its bottom communicate with at least one supply passage extending through the apex portions of the inverted hollow cones, the liquid which has been supplied can be dreained entirely through such supply passage during backwashing. It will be understood that the slurry can be supplied to a filtering package through the passages between the filtering package and the partition which engages the bottom of the package and may be supplied to the passages adjacent to the outer rim of the partition so that an improved distribution of the flowing fluid over the filtering package will be ensured.
The passages between the filtering packages and the partitions must ensure a satisfactory supply of the solids-laden fluid to all parts of the filtering package and a satisfactory withdrawal of filtrate from all parts of the filtering package. These requirements can be met in a convenient manner by defining the passages between each filter package and the adjacent partitions by ribs provided on those surfaces of the partitions which engage said package to define an array of passages on the entire surface of the filtering package so that the passages are open to the filtering package on a large area.
Because the filtering material of each filtering package is particulate and consists, e.g., of a powder, the package is easily deformable and the filtering material must be deformed to the proper shape before it is placed into the pressure vessel. This can be accomplished by mixing the filtering material with a soluble, preferably water-soluble binder, deforming the resulting mixture to form a body having a desired shape and then enclosing this body by the sheath. Such a shaped body can be handled in a convenient manner and can be properly fitted between two conforming partitions in the pressure vessel. When the filtering package has been subjected to the pressure required to hold the package in position, the binder must be dissolved out of the filtering material. This can easily be effected by washing the filtering packages with a suitable solvent.
Alternatively, filtering packages having a proper shape can be obtained by stretching the filtering package consisting of the filter material and the enclosing sheath on a dimensionally stable shaping cone so that the filtering package will be held in the shape which is determined by the shaping cone. When the filtering package stretched on the shaping cone has been placed into the pressure vessel, the shaping cone must be removed from the top of the package and must be replaced by the partition having a similar shape. To that end, the filtering package supported by the next lower partition, which has previously been installed in the pressure vessel, must be relieved from the forces retaining the package on the shaping cone. This can be accomplished if the shaping cone is hollow and can be torn open at least approximately along a generatrix of its conical surface. As the conical surface of the hollow shaping cone is torn open, the diameter of the shaping cone can be decreased and the shaping cone can be pulled from the filtering package without an exertion of forces on the package.